TECHNICAL FIELD
[0001] The present invention relates to a high strength steel sheet usable as a steel sheet
for automobiles and transport airplanes, and more specifically to a steel sheet having
a tensile strength of 1180 MPa or more.
BACKGROUND ART
[0002] In order to attain higher fuel economy in automobiles, transport airplanes, etc.,
it is desired to reduce an empty weight of an automobile or transport airplane. A
technique of using a high strength steel sheet and reducing a thickness thereof is
effective for the weight reduction. In particular, automobiles are required to ensure
collision safety. For example, structural components such as a pillar, and reinforcing
components such as a bumper and an impact beam, are required to further increase the
strength thereof. However, in general, as the strength of a steel sheet is increased,
ductility will be deteriorated, resulting in poor workability. Therefore, there is
a need for a steel sheet capable of satisfying both high strength and high ductility.
[0003] As a steel sheet having both high strength and high ductility, great interest has
been shown in a TRIP (Transformation Induced Plasticity) type steel sheet. As one
example thereof, there has been known a TBF steel sheet which comprises: bainitic
ferrite as its parent phase; and retained austenite (hereinafter occasionally denoted
as "retained γ") (see, for example, the following Non-Patent Document 1). In the TBF
steel sheet, high strength is obtained based on hard bainitic ferrite, and excellent
ductility is obtained based on fine retained γ existing in boundaries of the bainitic
ferrite.
[0004] Meanwhile, a steel sheet for use in automobiles and transport airplanes is also
required to be resistant to the occurrence of delayed fracture due to hydrogen embrittlement
(hereinafter referred to occasionally as "hydrogen embrittlement resistance"). The
delayed fracture means a phenomenon that hydrogen generated in a corrosive environment
or hydrogen in the atmosphere diffuses into defective areas, such as dislocations,
holes and grain boundaries, in the steel sheet, to embrittle the defective areas and
cause deterioration in ductility and rigidity of the steel sheet, and thereby fracture
will occur under a condition that static stress causing no plastic deformation is
applied to the steel sheet.
[0005] As a technique for improving hydrogen embrittlement resistance of the TBF steel sheet
comprising retained γ, the following Patent Documents 1 to 5 have been known. Among
them, the Patent Document 1 discloses a technique for improving hydrogen embrittlement
resistance of a high-strength thin steel sheet which comprises a main phase consisting
of bainite and bainitic ferrite, and a second phase consisting of austenite, with
the remainder being ferrite and/or martensite, and has a tensile strength of 800 MPa
or more. This Document includes a description mentioned that, in order to improve
the hydrogen embrittlement resistance, the strength and composition of the steel sheet
are adjusted to control a deposit serving as a hydrogen trap site, and the composition
of the steel sheet is adjusted to reduce a rate of hydrogen penetration into the steel
sheet.
[0006] The Patent Documents 2 to 5 disclose techniques which were previously proposed by
the applicant of this application. Metallographic structures of steel sheets disclosed
in each of these Documents comprise 1 area% or more of retained γ, and 80 area% or
more of a total of bainitic ferrite and martensite. These Documents include a description
mentioned that the parent phase of the steel sheet may be formed in a two-phase structure
of bainitic ferrite and martensite to reduce origins of intergranular fracture, and
retained γ is formed in a lath-like configuration to enhance a hydrogen trapping capability
to allow hydrogen to become harmless so as to improve the hydrogen embrittlement resistance.
[0007] EP 1 676 932 A1 disclosed a high strength thin steel sheet having high hydrogen embrittlement resisting
property wherein the purpose mentioned in that document is to provide a high strength
thin steel sheet that has a high hydrogen embrittlement resisting property. In order
to achieve the above purpose, a high strength thin steel sheet having a high hydrogen
embrittlement resisting property comprises:
residual austenite; 1 % by area or more in proportion to the entire structure;
bainitic ferrite and martensite: 80 % or more in total; and ferrite and martensite:
80 % or more in total; and ferrite and pearlite: 9 % or less (may be 0 %) in total,
while the mean axis ratio (mayor axis/ minor axis) of said residual austenite grains
is 5 or higher, and the steel has tensile strength of 1180 MPa or higher.
[0008] The Japanese document with the publication number
01-272720 discloses a high strength and ductility steel sheet with composite structure and
ensuring superior spot weldability by specifying the contents of C, Si, Mn, etc. and
carrying out properly controlled hot rolling, continuous annealing and cooling. A
steel slab consisting of 0.12 - 0.30 % C, 1.5 - 3.0 % Si, 1.1 - 2.4 % Mn, 0.01-0.1
% Nb, < 0.005 % S, 0.01 - 0.06 % sol. Al and the balance Fe with inevitable impurities
is hot rolled at a finishing temp. of the Ar
3 point or above. The resulting steel sheet is coiled at ≤ 600°C, cold rolled and subjected
to continuous annealing including holding in the austenite-ferrite two-phase range
of the Ac
1 point+30°C-the Ac
3 point for ≥ 4 min. It is slowly cooled to 500 - 800°C at 5 - 30 °C/sec cooling rate,
rapidly cooled to 350 - 450°C at ≥ 70°C/sec cooling rate, held at 350 - 450°C for
1 - 5 min and cooled to room temp. at ≥ 2°C/sec cooling rate. The composite structure
consisting of martensite, bainite, ferrite and retained austenite is formed.
[0009] EP 1 676 933 A1 disclose a high strength thin steel sheet having high hydrogen embrittlement resisting
property and high workability. The high strength thin steel sheet having high hydrogen
embrittlement resisting property has metallurgical structure after stretch forming
process to elongate 3 %, which comprises:
1 % more residual austenite;
80 % or more in total of bainitic ferrite and martensite; and 9 % or less (may be
0 %) in total of ferrite and pearlite in terms of proportion of area to the entire
structure, wherein the mean axis ratio (mayor axis/minor axis) of the residual austenite
grains is 5 or higher or
1 % or more residual austenite in terms of proportion or area to the entire structure;
mean axis ratio (mayor axis/minor axis) of the residual austenite grains is 5 or higher;
mean length of minor axes of the residual austenite grains is 1 µm or less;
minimum distance between the residual austenite grains is 1 µm or less; and
the steel has tensile strength of 1180 MPa or higher.
[0010] The steel sheet for automobiles and transport airplanes is required to satisfy both
high strength and high ductility, as mentioned above. Particularly as for strength,
it has recently been required to satisfy a tensile strength of 1180 MPa or more. However,
if the tensile strength is increased to 1180 MPa or more, the delayed fracture due
to hydrogen embrittlement is more likely to occur. Therefore, in the Patent Documents
2 to 4, the applicant disclosed and proposed a technique intended for a high strength
steel sheet having a tensile strength of 1180 MPa or more and designed to improve
the hydrogen embrittlement resistance, and obtained a certain level of effect. However,
there is a need for further improving the hydrogen embrittlement resistance.
LIST OF PRIOR ART DOCUMENTS
[PATENT DOCUMENTS]
[NON-PATENT DOCUMENTS]
SUMMARY OF THE INVENTION
[0013] The present invention has been made in view of the above circumstances, and an object
thereof is to provide a high strength steel sheet having a tensile strength of 1180
MPa or more while ensuring excellent hydrogen embrittlement resistance. It is another
object of the present invention to provide a method of producing the high strength
steel sheet.
[0014] According to one aspect of the present invention, there is provided a high strength
steel sheet having excellent hydrogen embrittlement resistance, wherein the steel
sheet has a tensile strength of 1180 MPa or more, and satisfies the following conditions:
with respect to an entire metallographic structure thereof, bainite, bainitic ferrite
and tempered martensite account for 85 area% or more in total; retained austenite
accounts for 1 area% or more; and fresh martensite accounts for 5 area% or less (including
0 area%).
[0015] According to another aspect of the present invention, there is provided a method
of producing a high strength steel sheet having excellent hydrogen embrittlement resistance.
The method comprises: a quenching step of cooling a steel sheet which contains, in
terms of mass%, C: 0.15 to 0.25%, Si: 1 to 2.5%, Mn: 1.5 to 3%, P: 0.015% or less,
S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.01% or less, and the balance of Fe and inevitable
impurities, and which has a temperature equal to or greater than an Ac
3 point, down to a temperature T1 satisfying the following formula (1), at an average
cooling rate of 10°C/sec or more; and a holding step of holding the steel sheet quenched
in the quenching step, at a temperature T2 satisfying the following formula (2), for
300 seconds or more.
[0016] These and other objects, features and advantages of the invention will become more
apparent from the following detailed description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
FIG. 1 is a photograph, as a substitute for a drawing, which depicts a metallographic
structure of a steel sheet of Sample No. 46 illustrated in Example.
FIG. 2 is a photograph, as a substitute for a drawing, which depicts a metallographic
structure of a steel sheet of Sample No. 38 illustrated in Example.
DESCRIPTION OF EMBODIMENTS
[0018] The inventors have been dedicated to studying for improving hydrogen embrittlement
resistance of a high strength steel sheet having a tensile strength of 1180 MPa or
more, with a focus on a metallographic structure of the steel sheet. As a result,
the inventors have accomplished the present invention based on the following findings,
after a steel sheet is formed to have a metallographic structure comprising a parent
phase consisting of a mixed structure of bainite, bainitic ferrite and tempered martensite,
and retained austenite as another structure, so as to enhance ductility on the premise
of ensuring a strength of 1180 MPa or more:
- (1) the hydrogen embrittlement resistance can be improved while maintaining the premise
of a high strength of 1180 MPa or more, by adequately controlling the metallographic
structure of the high strength steel sheet, particularly, to suppress fresh martensite
to 5 area% or less; and
- (2) the fresh martensite can be suppressed to 5 area% or less by adequately controlling
conditions for quenching and conditions for holding after the quenching to form fresh
martensite during the quenching and transform the fresh martensite into tempered martensite
through tempering so as to reduce fresh martensite to be newly formed during the holding.
The present invention will now be described in detail.
[0019] To begin with, types of metallographic structure characterizing the steel sheet of
the present invention will be described. In the present invention, the term "fresh
martensite" means a crystal grain in which no iron-based carbide appearing in white
exists, among a large number of crystal grains which appear in gray when a nital-etched
steel sheet surface is subjected to metallographic observation using a scanning electron
microscope. On the other hand, a crystal grain in which iron-based carbide exists
is defined as "bainite, bainitic ferrite or tempered martensite" and distinguished
from the "fresh martensite". The "fresh martensite" will hereinafter be occasionally
denoted as "F/M".
[0020] How the "fresh martensite" and the "bainite, bainitic ferrite or tempered martensite"
are distinguished from each other in an SEM photograph will be specifically described
using a photograph as a substitute for a drawing.
[0021] FIG. 1 is a photograph, as a substitute for a drawing, which depicts a metallographic
structure of a steel sheet of Sample No. 46 illustrated in Example described below,
and FIG. 2 is a photograph, as a substitute for a drawing, which depicts a metallographic
structure of a steel sheet of Sample No. 38 illustrated in the Example. When a nital-etched
steel sheet surface is subjected to observation using a scanning electron microscope,
an aggregate of gray crystal grains is observed in each photograph. In the photograph
illustrated in FIG. 1, as well as a crystal grain including a white point or a white
line composed of a linear array of continuously connected white points, a crystal
grain almost devoid of the white point or the white line is observed. On the other
hand, in the photograph illustrated in FIG. 2, a large number of crystal grains each
including the white point or the white line are observed, but a crystal grain almost
devoid of the white point or the white line is not observed. A result of composition
measurement of the white point (or the white line) showed that it is Fe-based carbide.
[0022] A difference between a crystal grain devoid of the white point or line, and a crystal
grain including the white point or line was checked. As a result, it was proven that
the crystal grain devoid of the white point or line is "fresh martensite" transformed
from austenite (in this specification, the term "austenite" is occasionally denoted
as "γ"), and the crystal grain including the white point or line is "bainite, bainitic
ferrite or tempered martensite" transformed from austenite.
[0023] Each of bainite, bainitic ferrite and tempered martensite is depicted as a gray crystal
grain including the white point or line, so that the three phases could not be distinguished
from each other.
[0024] A specific feature of the steel sheet according to the present invention will be
described below. The steel sheet of the present invention is characterized in that,
with respect to an entire metallographic structure thereof, bainite, bainitic ferrite
and tempered martensite account for 85 area% or more in total, as a parent phase,
and retained austenite accounts for 1 area% or more, as other structure, wherein fresh
martensite is suppressed to 5 area% or less (including 0 area%).
[0025] The parent phase consisting of bainite, bainitic ferrite and tempered martensite
makes it possible to enhance ductility, and the retained austenite makes it possible
to further enhance the ductility.
[0026] The largest feature of the steel sheet of the present invention is that fresh martensite
(F/M) is suppressed to 5 area% or less. The reason for setting this range will be
described in connection with a research process.
[0027] There has been known a technique of holding a steel sheet after quenching at a given
temperature to cause bainite transformation so as to produce a high strength steel
sheet, wherein it is considered that an effective way of obtaining higher strength
is to perform the holding step at a temperature as low as possible. Therefore, in
order to obtain higher strength of a TBF steel sheet, the production is performed
at a low holding temperature. As a result, hydrogen embrittlement resistance was significantly
deteriorated. Through various studies on this reason, it was proven that F/M is formed
in a steel sheet produced at a low holding temperature, and the hydrogen embrittlement
resistance is caused by the F/M. As the holding temperature is set to a lower value,
a diffusion speed of C becomes lower, so that the bainite transformation becomes less
likely to occur, and an austenite phase which has not been transformed during the
holding is transformed to form F/M, in the course of cooling to room temperature after
completion of the holding. Further, respective hydrogen embrittlement resistances
of a steel sheet formed with F/M and a steel sheet formed with no F/M were evaluated.
As a result, it was proven that the steel sheet formed with no F/M is more improved
in hydrogen embrittlement resistance than the steel sheet formed with F/M.
[0028] Then, the inventors studied a relationship between an amount of formation of F/M
and the hydrogen embrittlement resistance, in a high strength steel sheet having a
tensile strength of 1180 MPa or more. As a result, it was proven that, if F/M falls
within 5 area% with respect to the entire metallographic structure of the steel sheet,
the hydrogen embrittlement resistance becomes excellent. F/M accounts preferably for
2 area% or less, most preferably for 0 area%.
[0029] The parent phase of the steel sheet of the present invention is a mixed structure
of bainite, bainitic ferrite and tempered martensite. The parent phase formed as such
a mixed structure makes it possible to improve ductility while maintaining the required
strength.
[0030] With respect to the entire metallographic structure, the mixed structure accounts
for 85 area% or more, preferably for 90 area% or more, in total. Bainite, bainitic
ferrite and tempered martensite cannot be distinguished from each other in an SEM
photograph. Thus, they are defined by a total amount of the mixed structure.
[0031] In addition to the mixed structure, the steel sheet of the present invention comprises
retained austenite (retained γ). Retained austenite is a structure necessary particularly
to enhance ductility. The retained γ is present between bainite laths and between
bainitic ferrite laths.
[0032] It is necessary that, with respect to the entire metallographic structure, the retained
γ accounts for 1 area% or more, preferably for 4 area% or more. An upper limit thereof
is, for example, about 13 area%.
[0033] The steel sheet of the present invention has a metallographic structure primarily
comprising a parent phase consisting of bainite, bainitic ferrite and tempered martensite,
and retained γ, wherein F/M is suppressed to 5 area% or less. The steel sheet may
additionally comprise other structure inevitably formed during production, within
a range where advantageous effects of the steel sheet are not spoiled. For example,
the other structure may include ferrite and pearlite. For example, with respect to
the entire metallographic structure, the other structure accounts preferably for 10
area% or less, more preferably for 5 area% or less.
[0034] The Patent Document 1 discloses a high-strength thin steel sheet which comprises
a main phase consisting of bainite and bainitic ferrite, and a second phase consisting
of austenite, with the remainder being ferrite and/or martensite, and has a tensile
strength of 800 MPa or more. However, a point of dividing martensite into tempered
martensite and F/M and suppressing an amount of F/M is not disclosed therein. The
steel sheet in which F/M is suppressed to 5 area% or less cannot be found in steel
sheets specifically disclosed in Example. As for the steel sheet disclosed in each
of the Patent Documents 2 to 5 by the applicant of this application, the metallographic
structure thereof overlaps that of the high strength steel sheet of the present invention,
in that bainitic ferrite and martensite account for 80 area% or more in total, and
retained γ accounts for 1 area% or more. However, the point of dividing martensite
into tempered martensite and F/M and suppressing an amount of F/M is not disclosed
in these Documents.
[0035] A composition of the high strength steel sheet of the present invention will be described
below. The composition of the high strength steel sheet of the present invention may
be adjusted to allow a tensile strength to become equal to or greater than 1180 MPa
based on an alloy composition commonly comprised of a steel sheet for automobiles
and transport airplanes. For example, the composition may satisfy the following conditions:
C: 0.15 to 0.25%; Si: 1 to 2.5%; Mn: 1.5 to 3%; P: 0.015% or less (except for 0%);
S: 0.01% or less (except for 0%); Al: 0.01 to 0.1%; and N: 0.01% or less (except for
0%). The reasons for setting the above ranges are as follows.
[0036] C (carbon) is an element which is useful for increasing the strength of a steel sheet.
In addition, C is an effective element for formation of retained γ. In view of bringing
out the above functions, a content of C is preferably set to 0.15% or more. The content
of C is set more preferably to 0.17% or more, still more preferably to 0.19% or more.
However, if C is excessively contained, weldability and corrosion resistance will
be deteriorated. Thus, the content of C is preferably set to 0.25% or less. More preferably,
the content of C is set to 0.23% or less.
[0037] Si (silicon) is an element which contributes to an increase in strength of steel,
as a solid solution strengthening element. In addition, Si is an element capable of
suppressing formation of carbide to effectively function to form retained γ. In view
of bringing out the above functions, a content of Si is preferably set to 1% or more.
The content of Si is set more preferably to 1.2% or more, still more preferably to
1.4% or more. However, if Si is excessively contained, a scale will be significantly
formed to cause a scale trace in a surface of a steel sheet, during hot rolling, so
that a surface texture is likely to become worse. Moreover, pickling performance is
likely to be deteriorated. Thus, the content of Si is preferably set to 2.5% or less.
The content of Si is set more preferably to 2.3% or less, still more preferably to
2% or less.
[0038] Mn (manganese) is an element capable of enhancing quenchability to contribute to
an increase in strength of a steel sheet. In addition, Mn is an effective element
for stabilizing austenite to form retained γ. In view of bringing out the above functions,
a content of Mn is preferably set to 1.5% or more. The content of Mn is set more preferably
to 1.7% or more, still more preferably to 2% or more. However, if Mn is excessively
contained, segregation will occurs, so that workability is likely to be deteriorated.
Thus, the content of Mn is preferably set to 3% or less. The content of Mn is set
more preferably to 2.8% or less, still more preferably to 2.6% or less.
[0039] P (phosphorus) is an element which is inevitably contained, and apt to promote intergranular
embrittlement through segregation at grain boundaries. Thus, the content of P is preferably
set to 0.015% or less. It is recommended to reduce the content of P as much as possible.
The content of P is set more preferably to 0.013% or less, still more preferably to
0.01% or less.
[0040] S (sulfur) is an element which is inevitably contained as with P, and apt to promote
a steel sheet to absorb hydrogen in a corrosive environment. Thus, the content of
S is preferably set to 0.01% or less. It is desirable to minimize the content of S.
Specifically, it is set more preferably to 0.008% or less, still more preferably to
0.005% or less.
[0041] Al (aluminum) is an element which functions as a deoxidizing agent. In view of bringing
out the function, a content of Al is preferably set to 0.01% or more. The content
of Al is set more preferably to 0.02% or more, still more preferably to 0.03% or more.
However, if Al is excessively contained, a large amount of inclusions such as alumina
will be formed in a steel sheet, so that workability is likely to be deteriorated.
Thus, the content of Al is preferably set to 0.1% or less. The content of Al is set
more preferably to 0.08% or less, still more preferably to 0.05% or less.
[0042] N (nitrogen) is an element which is inevitably contained. If N is excessively contained,
a nitride will be formed, which causes deterioration in workability. Particularly,
in cases where B (boron) is contained in steel, N is combined with B to form a BN
precipitate, which hinders a quenchability enhancing function of B. Thus, the content
of N is preferably set to 0.01% or less. The content of N is set more preferably to
0.008% or less, still more preferably to 0.005% or less.
[0043] The steel sheet of the present invention satisfies the above composition condition,
and the remainder is iron and inevitable impurities.
[0044] As other element, the steel sheet of the present invention may contain:
- (A) Cr: 1% or less (except for 0%) and/or Mo: 1% or less (except for 0%);
- (B) B: 0.005% or less (except for 0%);
- (C) Cu: 0.5% or less (except for 0%) and/or Ni: 0.5% or less (except for 0%);
- (D) Nb: 0.1% or less (except for 0%) and/or Ti : 0.1% or less (except for 0%); and/or
- (E) one or more selected from the group consisting of Ca: 0.005% or less (except for
0%), Mg: 0.005% or less (except for 0%) and REM: 0.01% or less (except for 0%).
[0045] The reasons for setting the above ranges are as follows.
- (A) Cr (chromium) and Mo (molybdenum) are elements each capable of enhancing quenchability
to function to increase the strength of a steel sheet. They may be used independently
or may be used in combination.
[0046] Cr is an element which has a function of increasing temper softening resistance,
and a function of suppressing a reduction in strength during tempering of F/M, so
that it effectively functions to obtain higher strength of a steel sheet. In addition,
Cr is an element capable of preventing hydrogen from penetrating into a steel sheet,
and contributing to improvement in hydrogen embrittlement resistance because a Cr-containing
precipitate serves as a hydrogen trapping site. In view of bringing out the above
functions, a content of Cr is preferably set to 0.01% or more. The content of Cr is
set more preferably to 0.1% or more, still more preferably to 0.3% or more. However,
if Cr is excessively contained, ductility and workability will be deteriorated. Thus,
the content of Cr is preferably set to 1% or less. The content of Cr is set more preferably
to 0.9% or less, still more preferably to 0.8% or less.
[0047] On the other hand, Mo is an element capable of stabilizing austenite to effectively
function to form retained γ. In addition, Mo has a function of preventing hydrogen
from penetrating into a steel sheet to improve the hydrogen embrittlement resistance.
In view of bringing out the above functions, a content of Mo is preferably set to
0.01% or more. The content of Mo is set more preferably to 0.05% or more, still more
preferably to 0.1% or more. However, if Mo is excessively contained, workability will
be deteriorated. Thus, the content of Mo is preferably set to 1% or less. The content
of Mo is set more preferably to 0.7% or less, still more preferably to 0.5% or less.
[0048] In cases where Cr and Mo are used in combination, a total content of Cr and Mo is
preferably set to 1.5% or less.
(B) B (boron) is an element capable of enhancing quenchability to effectively function
to increase the strength of a steel sheet. In view of bringing out the function, a
content of B is preferably set to 0.0002% or more. The content of B is set more preferably
to 0.0005% or more, still more preferably to 0.001% or more. However, if B is excessively
contained, hot workability will be deteriorated. Thus, the content of B is preferably
set to 0.005% or less. The content of B is set more preferably to 0.003% or less,
still more preferably to 0.0025% or less.
(C) Cu (copper) and Ni (nickel) are elements each capable of suppressing generation
of hydrogen causing hydrogen embrittlement, and preventing the generated hydrogen
from penetrating into a steel sheet, so that they have a function of enhancing the
hydrogen embrittlement resistance. In other words, Cu and Ni are elements each capable
of enhancing corrosion resistance of a steel sheet itself, and preventing generation
of hydrogen due to corrosion of a steel sheet. In addition, these elements have a
function of promoting formation of α-FeOOH, as with Ti described below. Based on promoting
the formation of α-FeOOH, it becomes possible to prevent generated hydrogen from penetrating
into a steel sheet, so that the hydrogen embrittlement resistance can be enhanced
even in a harsh corrosive environment. In view of bringing out the above functions,
a content of Cu or Ni is set preferably to 0.01% or more, more preferably to 0.05%
or more, still more preferably to 0.1% or more. However, if Cu or Ni is excessively
contained, workability will be deteriorated. Thus, the content of Cu or Ni is set
preferably to 0.5% or less, more preferably to 0.4% or less, still more preferably
to 0.3% or less. One of the Cu and Ni may be added singularly to bring out the above
functions. In order to make it easy to develop the functions, it is preferable to
use Cu and Ni in combination.
(D) Nb (niobium) and Ti (titanium) are elements each functioning to make crystal grains
smaller to increase the strength and rigidity of a steel sheet. They may be used independently
or may be used in combination.
[0049] In view of bringing out the function of Nb, a content of Nb is preferably set to
0.005% or more. The content of Nb is set more preferably to 0.01% or more, still more
preferably to 0.03% or more. However, even if Nb is excessively contained, the advantageous
effect will be saturated, and a large amount of Nb precipitate will be formed, which
causes deterioration in workability. Thus, the content of Nb is preferably set to
0.1% or less. The content of Nb is set more preferably to 0.9% or less, still more
preferably to 0.08% or less.
[0050] On the other hand, Ti is an element which has a function of promoting the formation
of an iron oxide (a-FeOOH) which is considered as a thermodynamically stable one having
protective performance, among rusts to be formed in the air, in addition to the above
function. Based on promoting the formation of α-FeOOH, it becomes possible to prevent
hydrogen from penetrating into a steel sheet, so that the hydrogen embrittlement resistance
can be sufficiently enhanced even in a harsh corrosive environment. In addition, the
formation of α-FeOOH makes it possible to suppress the formation of β-FeOOH which
would otherwise be formed particularly in a chloride environment to cause a negative
effect on corrosion resistance (and thus hydrogen embrittlement resistance), so that
the hydrogen embrittlement resistance is further enhanced. Further, Ti is an element
which has a function of forming TiN to fix N in steel so as to effectively bring out
the quenchability enhancing effect from the addition of B. In view of bringing out
the above functions, a content of Ti is preferably set to 0.005% or more. The content
of Ti is set more preferably to 0.01% or more, still more preferably to 0.03% or more.
However, if Ti is excessively contained, a large amount of carbonitride will be precipitated,
which is likely to cause deterioration in workability and hydrogen embrittlement resistance.
Thus, the content of Ti is preferably set to 0.1% or less. The content of Ti is set
more preferably to 0.09% or less, still more preferably to 0.08% or less.
[0051] In cases where Nb and Ti are used in combination, a total content of Nb and Ti is
preferably set to 0.15% or less.
[0052] (E) Ca (calcium), Mg (magnesium) and REM (rare earth metals) are elements each capable
of preventing a hydrogen-ion concentration in surface-contacting atmosphere from being
increased due to corrosion of a surface of a steel sheet, and suppressing a lowering
in pH in the vicinity of the surface of the steel sheet to enhance corrosion resistance
of the steel sheet. In addition, these elements have a function of spheroidizing a
sulfide in steel to enhance workability. In view of bringing out the above functions,
a content of Ca, Mg or REM is set preferably to 0.0005% or more, more preferably to
0.001% or more, still more preferably to 0.003% or more.
However, if Ca, Mg or REM is excessively contained, workability will be deteriorated.
Thus, the content of Ca or Mg is preferably set to 0.005% or less. The content of
REM is set preferably to 0.01% or less, more preferably to 0.008% or less. One of
the Ca, Mg and REM may be contained singularly. Alternatively, two arbitrarily selected
from them may be contained, or all of the three elements may be contained.
[0053] In the present invention, the REM (rare earth metals) means elements including lanthanoid
(15 types of elements from La to Ln), Sc (scandium) and Y (yttrium). Among these elements,
it is preferable to contain at least one element selected from the group consisting
of La, Ce and Y, and it is more preferable to contain La and/or Ce.
[0054] The steel sheet of the present invention contains the above elements, and may additionally
contain any other element (such as Pb, Bi, Sb and/or Sn) within a range where advantageous
effects of the present invention are not spoiled.
[0055] A method for producing the steel sheet of the present invention will be described
below. As described above, a technique of holding a steel sheet at a low temperature
after quenching may be used for producing a high strength steel sheet, and a technique
of increasing a holding time may be used for completing the bainite transformation
during holding at a low temperature, to suppress the formation of F/M. However, as
a prerequisite to increasing the holding time, it is necessary to make a facility
longer, which leads to an increase in cost of the facility. Moreover, if the holding
time is increased, productivity will be deteriorated.
[0056] As a result of studies, the inventors have found that a metallographic structure
of a steel sheet can be adequately controlled while suppressing the formation of E/M,
by: subjecting steel satisfying the aforementioned composition condition to hot rolling
in a conventional manner and to cold rolling according to need; heating the rolled
steel sheet up to a temperature equal to or greater than an Ac
3 point; cooling the heated steel sheet down to a temperature T1 satisfying the following
formula (1), at an average cooling rate of 10°C/sec or more to quench the steel sheet
(quenching process); and holding the cooled steel sheet at a temperature T2 satisfying
the following formula (2), for 300 seconds or more (holding process). In the following
description, the holding time at the temperature T2 will be occasionally denoted as
"t3".
[0057] Specifically, a steel sheet is heated up to a temperature equal to or greater than
the Ac
3 point to form a metallographic structure thereof into single-phase austenite. Then,
the heated steel sheet is quenched in such a manner that it is supercooled down to
a temperature T1 satisfying the formula (1), at an average cooling rate of 10°C/sec
or more, so that a transformation from austenite to ferrite is suppressed to allow
the metallographic structure of the steel sheet to be formed as a mixed structure
of austenite and F/M.
[0058] Then, the steel sheet having the mixed structure is held at a temperature T2 satisfying
the formula (2), to allow the austenite in the mixed structure to be transformed to
bainite (or bainitic ferrite). During the holding, bainite transformation of the supercooled
austenite is completed. This makes it possible to prevent the formation of F/M during
cooling to room temperature after the holding. In addition, during the holding, F/M
can be transformed to tempered martensite. The holding process at the temperature
T2 has to be continued for 300 seconds or more. Because the holding time is required
to complete the bainite transformation and to increase a carbon concentration in the
austenite based on diffusion of carbon caused by the bainite transformation, so as
to allow stable retained γ to be formed even at room temperature.
[0059] In the holding process (holding step) of the present invention, a part of the austenite
is transformed to F/M. However, based on a combination of the supercooling to the
temperature T1 and the holding at the temperature T2 for a long time, an amount of
the formation of F/M is suppressed to 5 area% or less. Specifically, during the quenching,
the heated steel sheet is supercooled down to a temperature T1 ranging from (Ms point
- 250°C) to Ms point, so that a part of the γ is transformed to F/M. Thus, an amount
of γ (an area ratio of austenite existing in the steel sheet to the entire metallographic
structure thereof) at the start of the holding process can be reduced to an amount
of γ formed when the steel sheet is heated up to the Ac
3 point or more. Therefore, although a part of γ is transformed to F/M during the holding
process of the present invention, an amount of the γ before the transformation is
originally small, so that an amount of formation of F/M can be reduced.
[0060] If the quenching is performed under a condition that a temperature at the end of
the cooling of the steel sheet heated up to the Ac
3 point or more is set to a value greater than the Ms point, and then the quenched
steel sheet is held at a low temperature, the metallographic structure during the
quenching is formed as single-phase γ. Thus, during the holding process, bainite (or
bainitic ferrite) and F/M will be formed from the single-phase γ. Therefore, an amount
of F/M to be contained in a finally obtained steel sheet will be increased to a value
greater than 5 area%.
[0061] Details of production conditions will be described below. In the present invention,
a steel sheet is heated up to the Ac
3 point or more. In cases where the heating temperature is below the Ac
3 point, even if a two-phase structure of ferrite and austenite is subjected to quenching
and then holding, an amount of γ at the start of the holding process becomes excessively
small, so that a total amount of bainite, bainitic ferrite and tempered martensite
to be contained in a finally obtained steel sheet cannot be ensured, resulting in
lack of strength. Moreover, if the amount of γ at the start of the holding process
is excessively small, the γ is likely to disappear during the holding process, which
causes no formation of retained γ and deterioration in ductility of the steel sheet.
Therefore, the heating temperature is set to the Ac
3 point or more. An upper limit of the heating temperature may be set to about 950°C.
[0062] An average cooling rate from a temperature equal to or greater than the Ac
3 point to a temperature T1 satisfying the formula (1) is set to 10°C/sec or more.
If the average cooling rate is less than 10°C/sec, ferrite and pearlite are formed
from austenite, so that a strength of 1180 MPa or more cannot be ensured. The average
cooling rate is set preferably to 15°C/sec or more, more preferably to 20°C/sec or
more. For example, an upper limit of the average cooling rate is set to about 50°C/sec.
[0063] A temperature T1 just after quenching from a temperature equal to or greater than
the Ac
3 point is set in a range of (Ms point - 250°C) to Ms point. If the cooling-end temperature
T1 is greater than the Ms point, bainitic ferrite and bainite will be formed from
high-temperature austenite, so that a dislocation density is relatively lowered. Moreover,
almost no F/M is formed at the end of the cooling, so that almost no tempered martensite
exists in a final metallographic structure. This causes a lack of strength of a steel
sheet. Therefore, an upper limit of the temperature T1 is set to the Ms point. Preferably,
the upper limit of the temperature T1 is set to (Ms point - 20°C). On the other hand,
the temperature T1 just after quenching from a temperature equal to or greater than
the Ac
3 point is below (Ms point - 250°C), a large amount of F/M will be formed from γ during
the quenching, and thereby an amount of γ will be relatively reduced. If an amount
of γ is excessively small, the γ will disappear during the holding process, which
precludes the formation of retained γ, resulting in deterioration of ductility. Therefore,
a lower limit of the temperature T1 is set to (Ms point - 250°C). Preferably, the
lower limit of the temperature T1 is set to (Ms point - 200°C).
[0064] After being cooled to the temperature T1, the steel sheet is held at a temperature
T2 ranging from (Ms point - 120°C) to (Ms point + 30°C), for 300 seconds or more.
If the holding temperature T2 is greater than (Ms point + 30°C), a bainite crystal
grain will be enlarged, and carbide precipitated in a steel sheet will be enlarged.
This causes deterioration in strength, so that a tensile strength of 1180 MPa or more
cannot be ensured. Therefore, an upper limit of the temperature T2 is set to (Ms point
+ 30°C). Preferably, the upper limit of the temperature T2 is set to (Ms point + 20°C).
On the other hand, if the holding temperature T2 is below (Ms point - 120°C), a progress
of the bainite transformation will become slower. Thus, austenite existing in an untransformed
state during the quenching remains in a product steel sheet as F/M formed during the
holding process, so that the hydrogen embrittlement resistance is deteriorated. Therefore,
a lower limit of the temperature T2 is set to (Ms point - 120°C). Preferably, the
lower limit of the temperature T2 is set to (Ms point - 110°C).
[0065] When a steel sheet is held at the temperature T2, the temperature may be kept constant
in a range of (Ms point - 120°C) and (Ms point + 30°C), or may be changed within the
range. The range of the temperature T1 partially overlaps the range of the temperature
T2. This means that the cooling-end temperature T1 may be identical to the holding
temperature T2. Specifically, in cases where the cooling-end temperature T1 is in
a range of (Ms point - 120°C) to Ms point, the temperature T2 is set to a value identical
to the temperature T1, and held at the temperature T1. Alternatively, within the range
of (Ms point - 120°C) to (Ms point + 30°C), the temperature T2 may be set to a value
greater than the cooling-end temperature T1, or may be set to a value less than the
cooling-end temperature T1.
[0066] If the holding time t3 at the temperature T2 is less than 300 seconds, the progress
of the bainite transformation will become insufficient. Thus, concentration of carbon
in austenite remaining in an untransformed state during the quenching is not sufficiently
promoted. Thus, even if the steel sheet is held at the temperature T2 and then cooled
down to room temperature, F/M will remain in a product steel sheet. Consequently,
an amount of F/M to be contained in a finally obtained steel sheet cannot be suppressed
to 5 area% or less, so that it becomes impossible to improve the hydrogen embrittlement
resistance. Therefore, the holding time t3 is set to 300 seconds or more. The holding
time t3 is set preferably to 500 seconds or more, more preferably to 700 seconds or
more.
[0067] An upper limit of the holding time is not particularly limited. However, if the holding
time is excessively increased, it is likely that productivity is deteriorated, and
retained γ cannot be formed due to precipitation of a sold solution of carbon in the
form of carbide, which causes deterioration in ductility, resulting in poor workability.
Therefore, it is desirable that the upper limit of the holding time is set to about
1500 seconds.
[0069] The technique of the present invention is suitably applied, particularly, to a thin
steel sheet having a sheet thickness of 3 mm or less.
[0070] The steel sheet of the present invention obtained in the above manner is suitably
usable as a raw material of a component requiring high strength, for example, a seat
rail, a body component such as a pillar or a reinforcement member, or a reinforcing
component such as a bumper or an impact beam.
[0071] Although the present invention will be more specifically described below based on
examples, it is understood that the examples are not intended to limit the present
invention, but may be implemented while being appropriately changed or modified within
a range conformable to the aforementioned and aftermentioned points. Therefore, such
changes and modifications should be construed as being included in the scope of the
present invention hereinafter defined.
EXAMPLES
[0072] Steel having each composition illustrated in the following Tables 1 and 2 (the balance
is Fe and inevitable impurities) was vacuum melted to produce a test slab. An Ac
3 point and an Ms point were calculated based on each composition illustrated in the
Tables 1 and 2 and the formulas (a) and (b). The result is illustrated in the following
Tables 3 and 4. In the Tables 3 and 4, respective values of (Ms point - 250°C), (Ms
point + 30°C) and (Ms point - 120°C) are illustrated together.
[0073] The obtained test slab was subjected to hot rolling and then cold rolling. Subsequently,
the rolled slab was subjected to continuous annealing to obtain a steel sheet (sample).
Specific conditions of each process are as follows.
[0074] After the test slab was held at 1250°C for 30 munities, the test slab was subject
to hot rolling in such a manner that a finish rolling temperature becomes 850°C. Then,
the rolled slab was cooled from the finish rolling temperature to a winding temperature
of 650°C at an average cooling rate of 40°C/sec. After winding the cooled slab, the
wound slab was held at the winding temperature (650°C) for 30 minutes, and then cooled
in air to room temperature to obtain a hot-rolled steel sheet having a sheet thickness
of 2.4 mm. The obtained hot-rolled steel sheet was subjected to pickling to remove
a surface scale, and then subjected to cold rolling at a cold reduction of 50% to
obtain a cold-rolled steel sheet having a sheet thickness of 1.2 mm. The obtained
cold-rolled steel sheet was heated up to each heating temperature (°C) illustrated
in the Tables 3 and 4, and then quenched in such a manner that it is cooled to each
temperature T1 (°C) at each average cooling rate illustrated in the Tables 3 and 4.
Subsequently, the cooled slab was subjected to continuous annealing in which the slab
is held at each constant temperature T2 (°C) for each holding time t3 (sec) illustrated
in the Tables 3 and 4, to obtain a steel sheet (sample).
[0075] Then, a metallographic structure and mechanical characteristics of the obtained sample
was checked in the following manner. Further, when it is ascertained that a specific
sample has a tensile strength of 1180 MPa or more as a result of checking the mechanical
characteristics of each sample, hydrogen embrittlement resistance of the specific
sample was checked in the following manner.
Observation of Metallographic Structures
[0076] Each sample was cut at a 1/4 position of the sheet thickness along a direction parallel
to a rolling direction to form a cut surface. The cut surface was subjected to grinding
and further electrolytic polishing, and subjected to etching. A metallographic structure
of the sample was checked by observing the etched surface using a scanning electron
microscope (SEM).
[0077] The electrolytic polishing was performed for 15 seconds in a wet process using a
solution "Struers A2 (trade name)" produced by Struers Inc. The etching was performed
by bringing the cut surface into contact with a solution "Struers A2 (trade name)"
produced by Struers Inc, for 1 second.
[0078] A photograph of a metallographic structure taken by the SEM was subjected to image
analysis to measure each of an area ratio of a parent phase (bainite, bainitic ferrite
and tempered martensite) and an area ratio of fresh martensite (F/M). A magnification
for the observation was set to × 4000, and a field of view of the observation was
set to about 50 µm × 50 µm.
[0079] The parent phase and the F/M were distinguished from each other based on whether
there is Fe-based carbide within a crystal grain. Specifically, a crystal grain in
which a white point (or a white line composed of a linear array of continuously connected
white points) was observed in the image analysis of the SEM photograph, was determined
to be bainite, bainitic ferrite or tempered martensite, and a crystal grain in which
no white point (or no white line) was observed in the image analysis of the SEM photograph,
was determined to be F/M. Then, an area ratio of each structure was measured. A composition
of the white point (or the while line) observed within a crystal grain was analyzed
by XDR (X-Ray Diffraction). As a result, it was Fe-based carbide.
[0080] A photograph (as a substitute for a drawing) which depicts a metallographic structure
of a steel sheet of Sample No. 46, a photograph (as a substitute for a drawing) which
depicts a metallographic structure of a steel sheet of Sample No. 38, are illustrated
in FIG. 1 and FIG. 2, respectively.
[0081] In a metallographic structure of each sample, an area ratio of retained γ was measured
by a saturation magnetization method. Specifically, a saturation magnetization (I)
of the sample, and a saturation magnetization (Is) of a standard sample subjected
to a heat treatment at 400°C for 15 hours, were measured. Then, a rate of an austenite
phase (Vγ) was calculated from the following formula, and the calculated rate was
used as an area ratio of retained γ. The measurement of the saturation magnetization
was performed at room temperature using a DC magnetization B-H characteristic automatic
recorder "model BHS-40" produced by Riken Denshi Co. Ltd., under a condition that
a maximum applied magnetization was set to 5000 (Oe).
[0082] An area ratio of other structure (ferrite, pearlite, etc.) was derived by subtracting
the above structures (bainite, bainitic ferrite, tempered martensite, F/M and retained
γ) from the entire metallographic structure (100 area%), and a type of structure was
specified by SEM observation.
Evaluation of Mechanical Characteristics
[0083] As mechanical characteristics of each sample, a tensile test was carried out using
a No. 5 test piece defined by JIS Z2201 to measure a yield strength (YS), a tensile
strength (TS) and an elongation (El). The test piece was cut out from the sample to
allow a longitudinal direction thereof to be aligned with a direction perpendicular
to the rolling direction. A result of the measurement is illustrated in the following
Tables 5 and 6. In the present invention, when the TS is 1180 MPa or more, the sample
is evaluated as high strength (OK), and, when the TS is less than 1180 MPa, the sample
is evaluated as lack of strength (NG).
Evaluation of Hydrogen Embrittlement Resistance
[0084] A 150 mm × 30 mm reed-shaped test piece was cut out from each sample to allow a longitudinal
direction thereof to be aligned with a direction perpendicular to the rolling direction,
and subjected to bending to allow a bended portion to have a curvature radius (R)
of 10 mm. Then, under a condition that the test piece was immersed in a 5% aqueous
solution of hydrochloric acid while being loaded with a stress of 1500 MPa (strain
is converted to stress using a strain gauge), a time before the occurrence of crack
was measured as hydrogen embrittlement resistance of the sample. In the present invention,
when the time before the occurrence of crack is 24 hours or more, the sample is evaluated
as excellent hydrogen embrittlement resistance (OK), and, when the time before the
occurrence of crack is less than 24 hours, the sample is evaluated as poor hydrogen
embrittlement resistance (NG). A result of the evaluation is illustrated in the Tables
5 and 6. In the Tables 5 and 6, when the hydrogen embrittlement resistance is excellent,
the result is represented by o. When the hydrogen embrittlement resistance is poor,
the time before the occurrence of crack is indicated.
[0085] The following can be considered from the Tables 5 and 6.
[0086] Each of the samples Nos. 1 to 40 has a tensile strength of 1180 MPa or more, and
excellent hydrogen embrittlement resistance.
[0087] In contrast, each of the samples Nos. 41 to 50 fails to satisfy both a tensile strength
of 1180 MPa or more, and excellent hydrogen embrittlement resistance. Specifically,
each of the samples Nos. 41 to 44, 49 and 50 has a tensile strength of less than 1180
MPa, i.e., fails to satisfy the requirement defined by the present invention. Further,
each of the samples Nos. 45 to 48 has a tensile strength of 1180 MPa or more, but
fails to improve hydrogen embrittlement resistance. Each of the samples Nos. 41 to
50 will be discussed below.
[0088] In No. 41, the heating temperature is less than the Ac
3 point, so that an amount of formation of ferrite is increased. As a result, an amount
of formation of austenite is reduced, and thereby an amount of formation of bainite,
bainitic ferrite and tempered martensite is reduced. This causes a lack of strength.
In No. 42, the average cooling rate from the heating temperature to the temperature
T1 is less than 10°C/sec. Thus, a large amount of ferrite is formed, and thereby an
amount of formation of bainite, bainitic ferrite and tempered martensite is reduced,
which causes a lack of strength. In No. 43, the cooling-end temperature T1 after the
holding is excessively high, i.e., fails to reach the Ms point, which causes a lack
of strength. In No. 44, the holding temperature T2 is excessively high, i.e., greater
than (Ms point + 30°C), which causes a lack of strength. In No. 45, the cooling-end
temperature T1 after the holding is excessively low, i.e., less than (Ms point - 250°C),
which causes poor elongation. Moreover, the holding temperature T2 is excessively
low, i.e., less than (Ms point - 120°C), which causes deterioration in hydrogen embrittlement
resistance. In Nos. 46 to 48, the holding time t3 is excessively short. Thus, the
bainite transformation is sufficiently progressed, and thereby a large amount of F/M
remains, which causes deterioration in hydrogen embrittlement resistance. In Nos.
49 and 50, the tensile strength is less than 1180 MPa, i.e., does not satisfy the
requirement defined by the present invention.
[Table 1]
|
COMPOSITION (mass%) |
No. |
C |
Si |
Mn |
P |
S |
Al |
N |
Nb |
Ti |
Cu |
Ni |
Cr |
Mo |
B |
Ca, Mg, REM |
1 |
0.20 |
1.50 |
2.5 |
0.009 |
0.004 |
0.045 |
0.005 |
- |
- |
- |
- |
- |
- |
- |
- |
2 |
0.16 |
1.50 |
2.9 |
0.008 |
0.003 |
0.044 |
0.005 |
- |
- |
- |
- |
- |
- |
- |
- |
3 |
0.18 |
1.75 |
2.6 |
0.008 |
0.003 |
0.043 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
4 |
0.24 |
1.15 |
2.5 |
0.008 |
0.002 |
0.045 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
5 |
0.21 |
1.35 |
2.6 |
0.008 |
0.004 |
0.045 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
6 |
0.21 |
2.00 |
2.5 |
0.009 |
0.002 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
7 |
0.23 |
1.54 |
1.6 |
0.008 |
0.003 |
0.042 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
8 |
0.21 |
1.55 |
2.0 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
9 |
0.21 |
1.50 |
2.8 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
10 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
11 |
0.20 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
0.05 |
- |
- |
- |
- |
- |
- |
- |
12 |
0.20 |
1.54 |
2.5 |
0.008 |
0.003 |
0.042 |
0.004 |
- |
0.06 |
- |
- |
- |
- |
- |
- |
13 |
0.22 |
1.50 |
2.4 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
0.2 |
0.2 |
- |
- |
- |
- |
14 |
0.21 |
1.50 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
0.5 |
- |
- |
- |
15 |
0.21 |
1.45 |
2.4 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
- |
- |
- |
- |
0.2 |
- |
- |
16 |
0.22 |
1.60 |
2.5 |
0.008 |
0.003 |
0.042 |
0.004 |
- |
0.03 |
- |
- |
- |
- |
0.0021 |
- |
17 |
0.20 |
1.52 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
0.08 |
0.15 |
0.11 |
- |
- |
- |
- |
18 |
0.22 |
1.43 |
2.3 |
0.008 |
0.003 |
0.041 |
0.004 |
0.05 |
- |
0.12 |
0.11 |
- |
- |
- |
- |
19 |
0.22 |
1.43 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
0.05 |
0.07 |
0.12 |
0.11 |
0.5 |
- |
- |
- |
20 |
0.21 |
1.50 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
0.05 |
0.07 |
0.12 |
0.12 |
- |
0.2 |
- |
- |
21 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
0.05 |
0.05 |
0.11 |
0.11 |
- |
- |
0.0013 |
- |
22 |
0.22 |
1.54 |
2.5 |
0.008 |
0.003 |
0.042 |
0.004 |
0.05 |
0.08 |
0.12 |
0.10 |
- |
- |
- |
- |
23 |
0.20 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
0.08 |
0.12 |
0.11 |
0.5 |
0.15 |
- |
- |
24 |
0.19 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
0.08 |
0.13 |
0.11 |
0.8 |
- |
0.0015 |
- |
25 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
0.04 |
0.05 |
- |
- |
0.5 |
0.2 |
- |
- |
[Table 2]
|
COMPOSITION (mass%) |
No. |
C |
Si |
Mn |
P |
S |
Al |
N |
Nb |
Ti |
Cu |
Ni |
Cr |
Mo |
B |
Ca, Mg, REM |
26 |
0.20 |
1.54 |
2.5 |
0.008 |
0.003 |
0.042 |
0.004 |
0.05 |
0.05 |
- |
- |
0.7 |
- |
0.0014 |
- |
27 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
0.07 |
- |
- |
0.7 |
0.13 |
0.0007 |
- |
28 |
0.19 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
0.035 |
- |
0.3 |
0.2 |
0.8 |
0.13 |
- |
- |
29 |
0.22 |
1.54 |
2.5 |
0.008 |
0.003 |
0.044 |
0.004 |
- |
0.03 |
0.28 |
0.25 |
0.4 |
- |
0.0025 |
- |
30 |
0.20 |
1.54 |
2.5 |
0.008 |
0.003 |
0.042 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
Ca:0.004, Mg: 0.005, REM:0.005 |
31 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
32 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
33 |
0.18 |
1.75 |
2.6 |
0.008 |
0.003 |
0.043 |
0.004 |
- |
- |
- |
- |
0.08 |
- |
- |
- |
34 |
0.24 |
1.15 |
2.5 |
0.008 |
0.002 |
0.045 |
0.004 |
- |
- |
- |
- |
0.03 |
- |
- |
- |
35 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
0.04 |
- |
- |
- |
0.4 |
- |
- |
- |
36 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
0.04 |
0.05 |
- |
- |
0.5 |
- |
- |
- |
37 |
0.18 |
1.50 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
0.04 |
0.05 |
0.28 |
0.25 |
0.9 |
- |
0.0011 |
- |
38 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
0.07 |
- |
- |
39 |
0.21 |
1.54 |
2.5 |
0.011 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
40 |
0.21 |
1.54 |
2.5 |
0.011 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
41 |
021 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
42 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
43 |
0.24 |
1.54 |
2.6 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
44 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
45 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
46 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
0.07 |
- |
- |
47 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
48 |
0.21 |
1.54 |
2.5 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
49 |
0.11 |
1.54 |
2.0 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
50 |
0.21 |
1.55 |
1.4 |
0.008 |
0.003 |
0.041 |
0.004 |
- |
- |
- |
- |
- |
- |
- |
- |
[Table 3]
No. |
Ac3 (°C) |
Ms (°C) |
Ms-250 (°C) |
Ms+30 (°C) |
Ms-120 (°C) |
HEATING TEMPERATURE(°C) |
AVERAGE COOLING RATE (°C/sec) |
T1 (°C) |
T2 (°C) |
t3 (sec) |
1 |
835 |
384 |
134 |
414 |
264 |
900 |
30 |
320 |
380 |
1300 |
2 |
832 |
389 |
139 |
419 |
269 |
900 |
30 |
300 |
310 |
1300 |
3 |
846 |
390 |
140 |
420 |
270 |
900 |
30 |
300 |
310 |
1300 |
4 |
810 |
365 |
115 |
395 |
245 |
900 |
30 |
300 |
300 |
1300 |
5 |
822 |
376 |
126 |
406 |
256 |
900 |
30 |
320 |
360 |
1300 |
6 |
854 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
380 |
1300 |
7 |
855 |
399 |
149 |
429 |
279 |
900 |
30 |
280 |
310 |
1300 |
8 |
849 |
395 |
145 |
425 |
275 |
900 |
30 |
280 |
310 |
1300 |
9 |
823 |
369 |
119 |
399 |
249 |
900 |
30 |
300 |
380 |
1300 |
10 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
280 |
270 |
1300 |
11 |
836 |
384 |
134 |
414 |
264 |
900 |
30 |
320 |
380 |
1300 |
12 |
859 |
384 |
134 |
414 |
264 |
900 |
30 |
300 |
370 |
1300 |
13 |
824 |
374 |
124 |
404 |
254 |
900 |
30 |
300 |
360 |
1300 |
14 |
825 |
370 |
120 |
400 |
250 |
900 |
30 |
320 |
350 |
1300 |
15 |
839 |
378 |
128 |
408 |
258 |
900 |
30 |
300 |
320 |
1300 |
16 |
845 |
374 |
124 |
404 |
254 |
900 |
30 |
310 |
330 |
1300 |
17 |
861 |
382 |
132 |
412 |
262 |
900 |
30 |
310 |
360 |
1300 |
18 |
827 |
379 |
129 |
409 |
259 |
900 |
30 |
300 |
330 |
1300 |
19 |
845 |
364 |
114 |
394 |
244 |
900 |
30 |
300 |
320 |
1300 |
20 |
862 |
373 |
123 |
403 |
253 |
900 |
30 |
280 |
300 |
1300 |
21 |
850 |
377 |
127 |
407 |
257 |
900 |
30 |
280 |
300 |
1300 |
22 |
859 |
373 |
123 |
403 |
253 |
900 |
30 |
300 |
340 |
1300 |
23 |
863 |
370 |
120 |
400 |
250 |
900 |
30 |
290 |
300 |
1300 |
24 |
857 |
373 |
123 |
403 |
253 |
900 |
30 |
290 |
300 |
1300 |
25 |
853 |
366 |
116 |
396 |
246 |
900 |
30 |
290 |
300 |
1300 |
[Table 4]
No. |
Ac3 (°C) |
Ms (°C) |
Ms-250 (°C) |
Ms+30 (°C) |
Ms-120 (°C) |
HEATING TEMPERATURE(°C) |
AVERAGE COOLING RATE (°C/sec) |
T1 (°C) |
T2 (°C) |
t3 (sec) |
26 |
847 |
372 |
122 |
402 |
252 |
900 |
30 |
290 |
300 |
1300 |
27 |
857 |
364 |
114 |
394 |
244 |
900 |
30 |
290 |
300 |
1300 |
28 |
824 |
369 |
119 |
399 |
249 |
900 |
30 |
290 |
290 |
1300 |
29 |
830 |
363 |
113 |
393 |
243 |
900 |
30 |
290 |
290 |
1300 |
30 |
835 |
384 |
134 |
414 |
264 |
900 |
30 |
320 |
380 |
1300 |
31 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
310 |
310 |
500 |
32 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
330 |
700 |
33 |
846 |
389 |
139 |
419 |
289 |
900 |
30 |
300 |
310 |
1300 |
34 |
810 |
364 |
114 |
394 |
264 |
900 |
30 |
300 |
300 |
1300 |
35 |
828 |
372 |
122 |
402 |
272 |
900 |
30 |
320 |
380 |
1300 |
36 |
847 |
370 |
120 |
400 |
270 |
900 |
30 |
320 |
380 |
1300 |
37 |
838 |
374 |
124 |
404 |
274 |
900 |
30 |
260 |
290 |
1300 |
38 |
835 |
377 |
127 |
407 |
257 |
900 |
30 |
300 |
300 |
1200 |
39 |
834 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
320 |
400 |
40 |
834 |
379 |
129 |
409 |
259 |
900 |
30 |
310 |
310 |
300 |
41 |
832 |
379 |
129 |
409 |
259 |
800 |
30 |
320 |
370 |
1300 |
42 |
832 |
379 |
129 |
409 |
259 |
900 |
5 |
300 |
370 |
1300 |
43 |
823 |
361 |
111 |
391 |
241 |
900 |
30 |
400 |
390 |
1300 |
44 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
420 |
1300 |
45 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
30 |
30 |
1300 |
46 |
835 |
377 |
127 |
407 |
257 |
900 |
30 |
300 |
300 |
60 |
47 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
380 |
100 |
48 |
832 |
379 |
129 |
409 |
259 |
900 |
30 |
320 |
320 |
250 |
49 |
873 |
443 |
193 |
473 |
323 |
900 |
30 |
320 |
380 |
1300 |
50 |
866 |
415 |
165 |
445 |
295 |
900 |
30 |
320 |
380 |
1300 |
[Table 5]
No. |
MICROSTRUCTURE (area%) |
CHARACTERISTICS |
PARENT PHASE |
F/M |
RETAINED γ |
OTHER |
YS(MPa) |
TS(MPa) |
EI(%) |
HYDROGEN EMBRITTLEMENT RESISTANCE |
1 |
90 |
0 |
10 |
0 |
812 |
1187 |
12 |
○ |
2 |
91 |
0 |
9 |
0 |
925 |
1222 |
11 |
○ |
3 |
92 |
0 |
8 |
0 |
941 |
1251 |
11 |
○ |
4 |
95 |
0 |
5 |
0 |
1021 |
1449 |
9 |
○ |
5 |
90 |
1 |
9 |
0 |
803 |
1182 |
12 |
○ |
6 |
89 |
0 |
11 |
0 |
830 |
1232 |
12 |
○ |
7 |
95 |
0 |
5 |
0 |
816 |
1189 |
9 |
○ |
8 |
96 |
0 |
4 |
0 |
844 |
1258 |
8 |
○ |
9 |
93 |
0 |
7 |
0 |
895 |
1285 |
8 |
○ |
10 |
96 |
0 |
4 |
0 |
889 |
1328 |
10 |
○ |
11 |
90 |
0 |
10 |
0 |
832 |
1220 |
11 |
○ |
12 |
91 |
0 |
9 |
0 |
821 |
1193 |
12 |
○ |
13 |
91 |
0 |
9 |
0 |
824 |
1200 |
12 |
○ |
14 |
92 |
0 |
8 |
0 |
843 |
1221 |
11 |
○ |
15 |
96 |
0 |
4 |
0 |
1032 |
1493 |
8 |
○ |
16 |
93 |
0 |
7 |
0 |
890 |
1310 |
9 |
○ |
17 |
90 |
0 |
10 |
0 |
826 |
1225 |
12 |
○ |
18 |
92 |
0 |
8 |
0 |
861 |
1235 |
11 |
○ |
19 |
94 |
0 |
6 |
0 |
1004 |
1454 |
9 |
○ |
20 |
96 |
0 |
4 |
0 |
1040 |
1557 |
9 |
○ |
21 |
95 |
0 |
5 |
0 |
1025 |
1490 |
10 |
○ |
22 |
91 |
0 |
9 |
0 |
889 |
1359 |
11 |
○ |
23 |
96 |
0 |
4 |
0 |
1029 |
1478 |
8 |
○ |
24 |
96 |
0 |
4 |
0 |
1008 |
1466 |
9 |
○ |
25 |
96 |
0 |
4 |
0 |
1043 |
1512 |
7 |
○ |
[Table 6]
No. |
MICROSTRUCTURE (area%) |
CHARACTERISTICS |
PARENT PHASE |
F/M |
RETAINED γ |
OTHER |
YS(MPa) |
TS(MPa) |
EI(%) |
HYDROGEN EMBRITTLEMENT RESISTANCE |
26 |
94 |
0 |
6 |
0 |
1003 |
1460 |
10 |
○ |
27 |
94 |
0 |
6 |
0 |
1055 |
1530 |
7 |
○ |
28 |
94 |
0 |
6 |
0 |
1034 |
1516 |
10 |
○ |
29 |
95 |
0 |
5 |
0 |
1029 |
1504 |
9 |
○ |
30 |
90 |
0 |
10 |
0 |
815 |
1188 |
12 |
○ |
31 |
90 |
0 |
10 |
0 |
863 |
1272 |
10 |
○ |
32 |
90 |
0 |
10 |
0 |
843 |
1221 |
11 |
○ |
33 |
93 |
0 |
7 |
0 |
953 |
1262 |
11 |
○ |
34 |
96 |
0 |
4 |
0 |
1034 |
1458 |
9 |
○ |
35 |
90 |
0 |
10 |
0 |
843 |
1235 |
11 |
○ |
36 |
90 |
0 |
10 |
0 |
854 |
1243 |
11 |
○ |
37 |
94 |
0 |
6 |
0 |
1020 |
1455 |
9 |
○ |
38 |
92 |
0 |
8 |
0 |
921 |
1373 |
10 |
○ |
39 |
88 |
2 |
10 |
0 |
865 |
1241 |
10 |
○ |
40 |
86 |
4 |
10 |
0 |
871 |
1259 |
10 |
○ |
41 |
70 |
0 |
10 |
FERRITE: 20 |
720 |
1051 |
15 |
- |
42 |
55 |
0 |
5 |
FERRITE: 40 |
682 |
920 |
12 |
- |
43 |
95 |
3 |
2 |
0 |
769 |
989 |
13 |
- |
44 |
88 |
4 |
8 |
0 |
742 |
1085 |
14 |
- |
45 |
93 |
6 |
1 |
0 |
915 |
1382 |
4 |
6 |
46 |
72 |
20 |
8 |
0 |
874 |
1259 |
9 |
5 |
47 |
77 |
14 |
9 |
0 |
841 |
1221 |
9 |
8 |
48 |
84 |
6 |
10 |
0 |
821 |
1203 |
11 |
10 |
49 |
86 |
0 |
14 |
0 |
646 |
906 |
20 |
- |
50 |
73 |
0 |
12 |
FERRITE: 15 |
731 |
1078 |
17 |
- |
[0089] As described above in detail, according to one aspect of the present invention, there
is provided a high strength steel sheet having excellent hydrogen embrittlement resistance,
wherein the steel sheet has a tensile strength of 1180 MPa or more, and satisfies
the following conditions: with respect to an entire metallographic structure thereof,
bainite, bainitic ferrite and tempered martensite account for 85 area% or more in
total; retained austenite accounts for 1 area% or more; and fresh martensite accounts
for 5 area% or less (including 0 area%).
[0090] In the steel sheet of the present invention, the metallographic structure of the
high strength steel sheet having a tensile strength of 1180 MPa or more is adequately
controlled to suppress an amount of formation of fresh martensite to 5 area% or less,
so that it becomes possible to enhance hydrogen embrittlement resistance of the steel
sheet.
[0091] A composition of a steel sheet exhibiting a tensile strength of 1180 MPa or more
is already widely known (see, for example, the Patent Documents 2 to 4). The present
invention is directed to such a high strength steel sheet, and designed to control
the metallographic structure in the above manner so as to achieve the object of further
enhancing the hydrogen embrittlement resistance.
[0092] For example, a particularly preferred composition of the high strength steel sheet
of the present invention comprises, in terms of mass%, C: 0.15 to 0.25%, Si: 1 to
2.5%, Mn: 1.5 to 3%, P: 0.015% or less, S: 0.01% or less, Al: 0.01 to 0.1%, N: 0.01%
or less, and the balance of Fe and inevitable impurities.
[0093] The composition of high strength steel sheet of the present invention may further
comprise, as other element, an element satisfying at least one of the following conditions
(A) to (E):
- (A) Cr: 1% or less (except for 0%) and/or Mo: 1% or less (except for 0%);
- (B) B: 0.005% or less (except for 0%);
- (C) Cu: 0.5% or less (except for 0%) and/or Ni: 0.5% or less (except for 0%);
- (D) Nb: 0.1% or less (except for 0%) and/or Ti : 0.1% or less (except for 0%); and
- (E) one or more selected from the group consisting of Ca: 0.005% or less (except for
0%), Mg: 0.005% or less (except for 0%) and REM: 0.01% or less (except for 0%).
[0094] According to another aspect of the present invention, there is provided a method
of producing a high strength steel sheet having excellent hydrogen embrittlement resistance.
The method comprises: a quenching step of cooling a steel sheet which consists of
any one of the above compositions and has a temperature equal to or greater than an
Ac
3 point, down to a temperature T1 satisfying the following formula (1), at an average
cooling rate of 10°C/sec or more; and a holding step of holding the steel sheet quenched
in the quenching step, at a temperature T2 satisfying the following formula (2), for
300 seconds or more.
[0095] The method of the present invention makes it possible to reliably produce a high
strength steel sheet having excellent hydrogen embrittlement resistance.
INDUSTRIAL APPLICABILITY
[0096] The high strength steel sheet of the present invention is suitably usable as a raw
material of a component requiring high strength, for example, a seat rail, a body
component such as a pillar or a reinforcement member, or a reinforcing component such
as a bumper or an impact beam, in an automobile.